imapc: Fix assert-crash after detecting that multiple expunged mail r…

…eappear

The first imapc_mailbox_set_corrupted() call started a reconnect. The second
call crashed with:

Panic: file imapc-client.c: line 414 (imapc_client_mailbox_reconnect): assertion failed: (!box->reconnecting)

src/lib-storage/index/imapc/imapc-sync.c

@@ -346,7 +346,7 @@ imapc_initial_sync_check(struct imapc_sync_context *ctx, bool nooped)
 			imapc_mailbox_set_corrupted(ctx->mbox,
 				"Expunged message uid=%u reappeared", ruid);
 			ctx->failed = TRUE;
-			rseq++;
+			return;
 		}
 	}
 }